1. Field of the Invention
Embodiments of the present invention relate generally to heat assisted magnetic recording. In particular, embodiments of the present invention relate to a method and apparatus for heat assisted magnetic recording using a patterned recording medium and a method of manufacturing a patterned recording medium used in heat assisted magnetic recording.
2. Related Art
Heat assisted magnetic recording (HAMR) generally refers to the concept of locally heating a recording medium to reduce the coercivity of the recording medium so that an applied magnetic field can more easily direct the magnetization of the recording medium during the temporary magnetic softening of the recording medium caused by the heat source. HAMR has been proposed as a means by which the recording density of hard disc drives may be extended to 1 Tb/in2 or higher. Current conventional hard disc drive technology is limited by the superparamagnetic limit, which causes the small magnetic grains needed for high density recording media to gradually lose their magnetization state over time due to thermal fluctuations. By using HAMR, the magnetic anisotropy of the recording medium, (i.e., its resistance to thermal demagnetization), can be greatly increased while still allowing the data to be recorded with standard recording fields. For example, a laser beam, acting as a heat source, heats an area on the recording medium (called an “optical hot spot”) that is to be recorded and temporarily reduces the anisotropy in just that area sufficiently so that the applied recording field is able to set the magnetic state of that area. After cooling back to the ambient temperature, the anisotropy returns to its high value and stabilizes the magnetic state of the recorded mark.
The main difficulty with HAMR has been discovering a technique that is able to conduct sufficient light energy into the recording medium to heat it by several hundred degrees, but only in the area that is desired to be recorded, which typically will have dimensions on the order of 25 to 50 nm if the recording density is 1 Tb/in2. If the optical hot spot is larger than this area, it will extend to neighboring bits and tracks on the recording medium, and by heating those areas as well, the data recorded in those areas will be erased. Confining the optical hot spot to an area that is much smaller than a wavelength of light, and well below a so-called “diffraction limit” that can be achieved by standard focusing lenses, is an area of study called “near field optics” or “near field microscopy.” Conventional techniques have been described for confining light to 20 nm spots or smaller. However, these techniques have not demonstrated a capability for delivering a substantial amount of optical power to the sample within that small spot.
Several drawbacks exist with the near field optics technique of confining the optical hot spot. In general, a near field optics system generates high thermal gradients that define a written bit in both the down track and cross track directions in the presence of a low gradient field that flips the magnetization within the optical hot spot. The thermal conductivity of the recording medium is high so as to generate high thermal gradients but this requires higher power from the laser. Thus, one drawback with the near field optical technique is the requirement of a device or method to concentrate the laser beam in the desired spot efficiently.
Another drawback with the near field optical technique is exposing only one track of the recording medium at a time to the optical hot spot, which causes the spot to be very small. This requires greater concentration of the laser beam resulting in reduced efficiency (i.e., the power in the recording medium divided by the total laser power). When the size of the optical hot spot is much less than the wavelength of the light, it is very difficult to obtain acceptable efficiency because most of the light reflects off the aperture even with cleverly designed electron plasma resonance structures. For example, a 1 Tb/Sq system with a bit aspect ratio of 4 (i.e., bpi/tpi=4) would need a spot size of about 60 nm. This is only 13% of the wavelength of blue light. Thus, the efficiency problem is severe.
An even greater disadvantage with the near field optics technique is the requirement to co-locate the optical hot spot and the write field of the read/write head. This arrangement greatly compromises the write field gradient that is obtainable from the read/write head so that the transition must be defined mainly by the thermal gradient. Also, from a design and manufacturing perspective, it is much difficult to optimize the magnetic field and thermal structures simultaneously in a single structure.
Therefore, the need arises for a HAMR system that can extend magnetic recording to higher density by thermally lowering the coercivity of the recording medium in an optical hot spot during a write operation and thus, achieve a higher coercivity recording medium with greater room temperature thermal stability for write operations. The need also arises for a HAMR system that confines the optical hot spot to a narrow region as not to heat adjacent tracks causing adjacent track erasures for many write operations and mitigates the problem associated with combing the magnetic and thermal write operations in the HAMR system.